Magnets are materials capable of generating a magnetic field, which have the property of attracting ferromagnetic substances such as metals like iron, nickel, cobalt or the like. Magnets are generally classified into permanent magnets and soft magnets. Most of the materials as magnetizer and electromagnets are soft magnets, the polarity thereof varies as the polarity of the magnetic field applied thereon changes; and permanent magnets, i.e. hard magnets, are magnets capable of keeping their magnetic properties for a long time, which are not easily demagnetized, and are not easily magnetized, either. Therefore, no matter in industrial production or in daily life, a hard magnet is one of the most commonly-used powerful materials.
Hard magnets can be classified into natural magnets and artificial magnets. Artificial magnets are the magnets that are produced by synthesizing the alloys of different materials to not only be capable of obtaining the effects identical to those of natural magnets (lodestone) but also be capable of increasing the magnetic force. Artificial magnets have occurred since the eighteenth century, but the process to produce materials having stronger magnetic properties is very slow. Late in 1930s, aluminum nickel cobalt magnets (AlNiCo) were produced, making the large-scale use of magnets possible. Afterwards, ferrite was produced in 1950s. In 1960s, the occurrence of rare earth permanent magnets has developed a new era for the application of magnets. The first generation is samarium cobalt permanent magnet SmCo5, and the second generation is precipitation hardening type of samarium cobalt permanent magnets Sm2Co17. Up to now, the third generation neodymium iron boron permanent magnets materials (NdFeB) have been developed. Although ferrite magnets remain the most commonly-used permanent magnets materials at present, the output value of neodymium iron boron magnets has greatly exceeded that of the ferrite permanent magnets materials. Neodymium iron boron magnets have been developed into a big industry.
Neodymium iron boron magnet is also referred to as neodymium magnet, has a chemical formula of Nd2Fe14B, is a type of artificial permanent magnet and is the permanent magnet having the strongest magnetic force till now. Their maximum magnetic energy product (BHmax) is at least 10 times higher than that of ferrite. When it is in a state of bare magnet, the magnetic force thereof can reach about 3500 gauss. Neodymium iron boron magnets have the advantages such as high cost performance, small volume, light weight, good mechanical properties and strong magnetic properties. Such advantage of high-energy density makes neodymium iron boron permanent magnets materials extensively applicable in modern industry and electronic technology, and they are called as king of magnets in magnetology. Therefore, how to expand applications of neodymium iron boron magnets has always been the focus that draws the continuing attention of the industry.
After several decades of development, the magnetic properties of sintered neodymium iron boron magnets have been constantly enhanced, wherein magnetic remanence Br and maximum magnetic energy product (BH)max have been close to the limit value. However, the actual coercive force of sintered NdFeB is only about 30% of the theoretical value. Therefore, increasing coercive force is critical for the enhancement of the comprehensive properties of sintered neodymium iron boron magnets. Currently, methods for enhancing coercive force are mainly achieved by means of enhancing coercive force by direct addition of heavy rare earth during smelting. However, these methods will evidently reduce magnetic remanence and magnetic energy product on the basis of enhancing the coercive force.
Therefore, how to find a more suitable method for enhancing coercive force, while maintaining magnetic remanence and maximum magnetic energy product at the same time, has always been the focus that draws the extensive attention of the research-and-development type manufacturers of neodymium iron boron magnets in the industry.